Even platinum is better than iridium in natural abundance and cost, nowadays, only iridium(III) emitters are used OLED display panels. Many remaining issues have to be solved before platinum(II) emitters can be used in OLED display panel production. Efficiency roll-off is one of the most important issues encounter by platinum(II) emitters. It is because platinum emitters adopt a square planar geometry and the platinum centers tends to come together, platinum(II) emitters usually have very high self-quenching constants (in the order of 108 dm3 mol−1 s−1 or more). Together with the long emission lifetime (near or longer than 10 μs), the devices fabricate from platinum(II) emitters usually have severe triple-triplet annihilation leading to serious efficiency roll-off.
Limited by the low emission quantum efficiency, OLEDs fabricated from platinum(II) emitter have shown related low efficiency. In the past decade, due to the improvement in emission quantum efficiency, the maximum device efficiency of 51.8 cd/A has been achieved by platinum(II) emitters [Appl. Phys. Lett. 91, 063508 (2007)]. However, the efficiencies of these device drops to less than 50% of the maximum values when the brightness increased to acceptable operation brightness say 1,000 cdm−2. This is not good for OLED applications.
Besides serious efficiency roll-off, OLEDs fabricated from emitters which tend to come together (in the other words: tends to self-aggregate/have high self-quenching constant) always have narrow doping window (devices with high efficiency and good color purity can only be obtained in a very narrow doping concentration range such as 1 wt. %-5 wt. %). As the fabrication systems in industry are much large than those in research institutes, making devices within such narrow doping window is not an easy task. As a result, platinum(II) materials are not yet been used in industry.
Some efforts have been made to deal with this issue. Bulky groups such as tert-butyl group(s) and non-planar phenyl group have been added to the emitters. However, they are not successful. In 2010, Che added tert-butyl group(s) in red-emitting platinum(II) material. [Chem. Eur. J. 2010, 16, 233-247]. However, close intermolecular stacking π-π interactions were still observed in the X-Ray crystal structure which means the problem cannot be resolved.
In the same year, Huo report a class of platinum(II) materials containing a non-planar phenyl ring, however excimer emission appears in doping concentration more than 4 wt. % and severe triplet-triplet annihilation was observed even in a device with a mix host, which means this approach cannot solve the problem [Inorg. Chem. 2010, 49, 5107-5119].
In 2013, Xie prepared new emitters containing two non-planar spiro-structures. [Chem. Commun. 2012, 48, 3854-3856] However, the devices fabricated by this emitter show serious efficiency roll-off of >50% which indicates adding non-planar group(s) may able to reduce roll-off.
In the same year, Che combined the two approaches and using a new, robust (O^N^C^N) ligand system to prepare new platinum(II) materials. In this approach, one of the emitters shows a wide doping window and slow efficiency roll-off [Chem. Commun. 2013, 49, 1497-1499]. However, the quenching constants of these materials are still high (minimum value: 8.82×10−7 dm3 mol−1 s−1) which made the maximum efficiency of the device only achieve 66.7 cd/A, whereas even the emission quantum efficiency of the device is 90%. Close or more than 100 cd/A should be obtained with this emission quantum efficiency if the quenching effect is resolved.